You are off to a good start by communicating. I know your feelings are pretty raw and you most likely feel threatened by this couple, but try and phrase your comments as "I feel you are putting this friendship before me" instead of telling her she is doing it. She cannot discount your feelings, but she can feel as if you are attacking her and become defensive if you are telling her she is doing something. Encourage her to talk about her "friends" and what makes them so special to her. She should reveal to you where the void is in her life and how you can fill it. It is wonderful that you both have stopped swinging for now. I believe it a good step to healing. However, try and reserve your feelings of never wanting to do this again until you are in a better place with your marriage. It is great to say you want to hold off for now, but she might feel as if you are trying to "punish" her by telling her you will never do this again.
Ok, I will step down from my psychobabble soap box.
Best of luck to you.
MandW